1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to the die closing unit of a plastics injection molding machine and to the structure of cast iron die carrier plates which, as part of the die closing unit, serve to support the two die halves of the injection molding die.
2. Description of the Prior Art
The injection molding of plastic precision parts, such as parts for precision instruments and the like, requires not only a high degree of accuracy in the guidance and alignment of the two die halves, but also an elevated die closing pressure during the injection of the part. In order to minimize weight deviations--a reflection of dimensional deviations --of the injection-molded parts over an extended period of time, it is also important that the wear on those surfaces which provide the support and guidance of the movable die half be kept to an absolute minimum.
Initially, the degree of accuracy achieved is a function of the accuracy with which the stationary and movable die carrier members of the die closing unit are machined and aligned on the machine base and, of course, also a function of the dimensional accuracy of the injection molding die itself.
A crucial factor in the maintenance of the initial accuracy of guidance and alignment and in the minimization of long-term wear, under the rhythmically applied high stress of the die closing pressure, in alternation with the die opening and closing movements, is the rigidity of the die closing unit. This rigidity is to a certain degree influenced by the rigidity of the machine base of the injection molding machine and by the manner in which the die closing unit is attached to the machine base.
It has been found that a relatively rigid die closing unit is obtainable with a structural configuration in which the movable die carrier member is supported and guided on four parallel horizontal tie rods which extend between the stationary die carrier member and a second stationary member in the form of a cylinder head plate, whereby the movable die carrier member is pushed closed by means of a single hydraulic cylinder assembly which is arranged in the center axis of the die closing unit and attached to the axially outer side of the cylinder head plate.
The result is a skeleton structure in which the four tie rods form longitudinal members at the four corners of a square prism, and the two stationary members serve as transverse connecting members at the two extremities of the prism. This type of die closing unit is known from the prior art and disclosed, for example, in German Patent No. 25 44 537 and in the corresponding U.S. Pat. No. 4,080,144.
Additional torsional stiffness is provided by a box-like machine base which has two parallel guide rails welded to the upper edges of its longitudinal side walls. The stationary die carrier member and the stationary cylinder head plate engage horizontal and vertical faces of the guide rails with angular flange formations. The two guide rails thus assure the precise axial alignment of the two stationary members. The two guide rails may also be used to provide additional support and guidance for the movable die carrier member, as suggested in the German Patent No. 31 40 740 and in the corresponding U.S. Pat. No. 4,453,912.
It has also been found that, even with four heavy tie rods, the elevated die closing pressure produces a minute relative separation of the two stationary members of the die closing unit, as the tie rods undergo elongation. In order to maintain a precise parallel alignment between the stationary and movable die carrier members, it has therefore also been suggested to clamp only the stationary die carrier member to the guide rails of the machine base and to arrange for the stationary cylinder head plate to be free to execute small longitudinal displacements on the guide rails. Such an arrangement is disclosed in my copending U.S. patent application Ser. No. 443,644 filed Nov. 22, 1982, now U.S. Pat. No. 4,530,655.
Careful observations and measurements made over an extended period of time on highly stressed die closing units have now revealed that, even with all the measures heretofore proposed for the purpose of eliminating any possible alignment distortions of the die carrier members, minute, not readily measurable rhythmic distortions do take place and that, although no wear takes place on the die closing unit and no weight deviations of the injection-molded parts are registered for a considerable length of time, such wear does takes place over the long run, and it appears to increase in a geometric progression.
Extensive experiments and tests have lead to the conclusion that this problem is connected with the stationary die carrier member which appears to undergo minute distortions, despite the fact that it has a heavy block-shaped body and is firmly bolted to the guide rails of the machine base.
The two die carrier members of this type of die closing unit have oppositely facing planar die mounting faces which extend over an area which is at least as large as the square (or rectangle) enclosed by four tangents to the most proximate points of each pair tie rods, and preferably over an area which is as large as the square (or rectangle) enclosed by four tangents to the most distant points of each pair of tie rods.
These die carrier members are normally of cast iron, at least the stationary die carrier member having interior cavities between parallel walls on its inner and outer axial sides, for the purpose of reducing its weight. The casting operation for these members requires a relatively high casting temperature which, on the other hand, must be kept low enough to minimize such undesirable side effects as gas adsorption, oxidation, cavity formation, grain coarsening and the like, including chemical reactions with the mold materials in the walls of the mold.
The rate of solidification and cooling within the die carrier casting differs in various regions and zones of the latter, depending on the wall thickness and on the rate at which the casting form removes heat from the casting. As a result, the molecular structure of the solidified cast iron is affected differently in different zones of the casting, and internal stresses remain in the cooled-off casting.
These molecular differences reflect themselves in nonuniform mechanical characteristics of the metal and, particularly, in an increased stress resistance and a reduced elongation of the surface zones of the casting --commonly known as the "casting skin"--as compared to the main body of the casting.
Following the cooling-off process, such a casting may be outwardly stable, inspite of the fact that its surface zones have solidified and hardened earlier than its interior zones. Thanks to the coherence and the high resistance of the casting skin, the latter may contain the interior stresses to such an extent that, initially at least, no noticeable distortions of shape occur in the casting. Problems tend to arise, however, when, in the course of machining operations on the die carrier casting, the casting skin is removed. The extent of which such distortion problems are encountered is related to the size and degree of continuity of the surface area over which the casting skin has been removed.
On the other hand, the removal of large portions of the casting skin through machining also results is a considerable reduction in the stiffness of the casting, i.e. in the mechanical resistance of the casting against deformation, especially against bending. This is due to the fact that the casting skin offers a higher stress resistance and less elongation than the remainder of the casting and the fact that the tensile and compressive stresses created by a bending load are highest in those regions of the casting which are farthest from the center--more precisely, the neutral bending axis--the casting.